I think the the humming is possibly pointing to a amplifier that is on its way out. That is one of the first symptoms. I wouldn't leave the sub on standby either until you know for sure what the issue is. It could start making popping noises and then blow (make a big sound) all together which could startle the whole the house especially at night.
Humming can also come from a ground loop which is fixable but they just don't magically appear and the humming is usually present when the HT is on. Ground loops usually come from adding new equipment, cables, room lighting or moving all the equipment to a different part of the house.
